New Zealand's highest-ranked fast cricket bowler returns for the final ODI against Sri Lanka tomorrow, but Trent Boult is not a cast iron guarantee to play at Bay Oval.

Left armer Boult, ranked third in the ODI game, has sat out the first four games of the series for a planned rest. His return is timely.

His regular new ball partner Tim Southee is nursing a sore left foot from Thursday's third match in Nelson, while fast-medium Doug Bracewell has been left out of the squad for a game Sri Lanka must win to square the series. A win for the tourists is off the books after rain washed out Saturday's fourth match in Nelson.

The ND swing man has had a messy summer. He was struggling for fitness for the first two tests in Australia before Christmas, due to a sore back. He came back with five wickets in Australia's second innings of the pink ball test in Adelaide but he's had problems keeping his pace up.

Assistant coach Craig McMillan is optimistic Boult will be ready to return tomorrow. "He's pretty good from what I've heard. The break has done him well," he said last night.

"We need to see how he goes at training tomorrow before we make a final decision on whether he plays or not.

"There's been a heavy workload on a couple of guys in the seam attack and it's important we get him back to full fitness because we know what a potent weapon he is when fully fit."

New Zealand's most effective bowler in the series has been Canterbury seamer Matt Henry, who took four wickets in each of the first two games in Christchurch, but was then expected to miss the rest of the series as rotation took place.

However he's back, given Southee's injury left New Zealand one seamer short.

McMillan said Sri Lanka deserved credit for the way they fought back into the series with a strong performance for an eight-wicket win at Nelson on Thursday.